1. Field of the Invention
The present invention relates to a load driver for driving a DC load connected to a DC power supply. The present invention further relates to a control method for driving the DC load connected to the DC power supply. Moreover, the present invention relates to a computer-readable recording medium on which a program is recorded that allows a computer to execute the control for driving the DC load.
2. Description of the Background Art
Hybrid vehicles and electric vehicles are now attracting considerable attention as they help the environment. Some hybrid vehicles are now commercially available.
The hybrid vehicle includes, as its power source, a DC power supply, an inverter and a motor driven by the inverter in addition to a conventional engine. Specifically, the engine is driven to generate power while a DC voltage from the DC power supply is converted into AC voltage by the inverter to rotate the motor by the AC voltage and accordingly generate power. The power source of the electric vehicle is a DC power supply, an inverter and a motor driven by the inverter.
Such a hybrid vehicle or electric vehicle is designed for example to include a motor driver as shown in FIG. 16. Referring to FIG. 16, motor driver 600 includes a DC power supply B, system relays SR1 and SR2, a capacitor C, a bidirectional voltage converter 410, and an inverter 420. Bidirectional voltage converter 410 includes a reactor L, NPN transistors Q10 and Q11, and diodes D10 and D11. Reactor L has one end connected to a power supply line of DC power supply B and the other end connected to an intermediate point between NPN transistors Q10 and Q11, i.e., between the emitter of NPN transistor Q10 and the collector of NPN transistor Q11. NPN transistors Q10 and Q11 are connected in series between a power supply line and a ground line. NPN transistor Q10 has its collector connected to the power supply line of inverter 420 while NPN transistor Q11 has its emitter connected to the ground line. Between the emitter and collector of NPN transistors Q10 and Q11 each, corresponding one of diodes D10 and D11 is provided to flow current from the emitter to the collector.
DC power supply B supplies a DC voltage to capacitor C when system relays SR1 and SR2 are made on. Capacitor C smoothes the DC voltage from DC power supply B to supply the smoothed DC voltage to bidirectional voltage converter 410. Bidirectional voltage converter 410 is controlled by a control unit (not shown) to boost the DC voltage from capacitor C in response to a period during which NPN transistor Q11 is kept on. Converter 410 then supplies the boosted DC voltage to inverter 420. Bidirectional voltage converter 410 is also controlled by the control unit to down-convert a DC voltage converted by inverter 420 to charge DC power supply B in regenerative power generation by a motor M.
Inverter 420 receives the DC voltage from bidirectional voltage converter 410 via a smoothing capacitor (not shown) and converts the DC voltage into an AC voltage under control by a control unit (not shown) to drive motor M. Further, in regenerative power generation mode by motor M, inverter 420 receives an AC voltage from motor M and converts the AC voltage into a DC voltage under control by the control unit to supply the DC voltage to bidirectional voltage converter 410. Motor M is driven by inverter 420 to generate predetermined torque. In addition, motor M serves as a regenerative generator to supply the generated AC voltage to inverter 420.
DC/DC converter 430 is located between bidirectional voltage converter 410 and DC power supply B to be connected to DC power supply B and receives the DC voltage from DC power supply B. DC/DC converter 430 is used for auxiliary equipment of the vehicle and down-converts the DC voltage from DC power supply B and supplies the down-converted DC voltage to an inverter (not shown) driving an air conditioner (not shown) provided in the hybrid or electric vehicle.
In motor driver 600, DC power supply B supplies the DC voltage to capacitor C when system relays SR1 and SR2 are made on, and then capacitor C smoothes the DC voltage to supply the smoothed voltage to bidirectional voltage converter 410 and DC/DC converter 430. Bidirectional voltage converter 410 boosts the DC voltage in response to a period during which NPN transistor Q11 is kept on and supplies the boosted DC voltage to inverter 420 via the smoothing capacitor (not shown). Inverter 420 converts the DC voltage into the AC voltage to drive motor M. Motor M generates predetermined torque. On the other hand, DC/DC converter 430 down-converts the DC voltage from capacitor C to supply the down-converted voltage to the inverter which drives the air conditioner.
In regenerative braking of the hybrid or electric vehicle, motor M generates the AC voltage to be supplied to inverter 420. Inverter 420 converts the AC voltage from motor M into the DC voltage to be supplied to bidirectional voltage converter 410. Bidirectional voltage converter 410 down-converts the DC voltage from inverter 420 to charge DC power supply B. In this way, motor driver 600 boosts the DC voltage from DC power supply B to drive motor M, and motor driver 600 also charges DC power supply B with the voltage generated by motor M in regenerative braking.
Alternatively, a hybrid or electric vehicle is designed to include a motor driver as shown in FIG. 17. Referring to FIG. 17, motor driver 700 has the same configuration as that of motor driver 600 except that a DC/DC converter 440 of motor driver 700 is connected to the output of bidirectional voltage converter 410.
DC/DC converter 440 receives a voltage which is boosted by bidirectional voltage converter 410 and down-converts the boosted voltage to charge an auxiliary buttery 450 (with output voltage of 12 V for example) which supplies electric power to such a control circuit as an ECU (Electrical Control Unit). Regarding the configuration as shown in FIG. 17, even if any abnormal event of DC power supply B, fuse blowing or any abnormal event of system relays SR1 and SR2 for example occurs, DC/DC converter 440 is supplied with a DC voltage generated by motor M1 and converted by inverter 420. In other words, even if any abnormal event occurs in the circuitry between bidirectional voltage converter 410 and DC power supply B, auxiliary buttery 450 for driving such a control circuit as ECU never becomes empty and thus the vehicle is prevented from being unable to move.
As for the conventional motor driver 600 in regenerative power generation, if DC power supply B is separated due to malfunction of system relays SR1 and SR2 or break, a voltage Vb appearing on the DC power supply B side of bidirectional voltage converter 410 increases resulting in a problem that an overvoltage is applied to DC/DC converter 430 which is a DC load.
In order to protect DC load system from the overvoltage, the withstand voltage of the DC load system should be enhanced which requires components with a high withstand voltage. Then, the overall cost cannot be reduced. Therefore, it is necessary to prevent the overvoltage from being applied to the DC load system in regenerative power generation if the DC power supply B is separated due to any reason.
As for the conventional motor driver 700, DC/DC converter 440 is connected to the output of bidirectional voltage converter 410. Then, a high withstand voltage is required and accordingly, the requirements of the specification of components are considerably severe. A resultant problem is that the configuration of the circuitry becomes complicated which leads to difficulty in reduction of the cost and size.